The present invention relates generally to a loudspeaker system, and more particularly, to a dynamic loudspeaker unit which includes a voice coil consisting of two windings and is designed to lower a reproducible threshold frequency in a bass zone or low sound frequency zone.
A prior art loudspeaker system of the type mentioned above is shown in FIG. 1. Principles of the loudspeaker system shown therein are as follows: It includes a dynamic loudspeaker in which a voice coil 1 consisting of a first winding 1a and a second winding 1b is coupled to a diaphragm 2 and the windings are positioned within a magnetic gap of a magnetic circuit. Between input terminals 3, 3', a first circuit including the first winding 1a and a second circuit including the second winding 1b are connected in parallel. To the first winding 1a of the first circuit, an LC series resonance circuit consisting of an inductor L and a capacitor C is connected in series. A resonance frequency of the LC series resonace circuit is selected to be approximately equal to the lowest resonance frequency f.sub.oc of the loudspeaker unit. Accordingly, at frequencies around the lowest resonance frequency f.sub.oc, currents will flow through not only the second winding 1b but also the first winding 1a, so that a sound pressure level of the loudspeaker at the frequency around the lowest resonance frequency f.sub.oc is increased, resulting that apparent resonance quality factor Q.sub.oc of the loudspeaker is increased.
A sound pressure characteristic A of the loudspeaker thus constructed is shown in FIG. 2 where a characteristic B of a loudspeaker having a voice coil consisting of a single winding is also shown for the purpose of comparison. Further, an impedance characteristic A of the former loudspeaker is shown in FIG. 3 together with a characteristic B of the latter one. It is understood from FIG. 3 that the impedance characteristic A is made flat over the entire sound frequency zone.
As noted above, in this loudspeaker system, the lowest resonance frequency f.sub.oc is not altered but only apparent Q.sub.oc is only changed. Therefore, in order that sound may be reproduced to a lower frequencies, it is necessary to increase the weight of a vibrating system, to thereby lower f.sub.oc. As a result, the sound pressure level will drop. Thus, in order to maintain the initial sound pressure level, a larger driving force, i.e., force coefficient Bl (B being a magnetic flux density in a magnetic gap and l being an effective length of a voice coil) is needed. If so designed, then Q.sub.oc is lowered and as a consequence, a wanted rise of the sound pressure level can not be obtained.
In addition, due to lowering of an impedance as shown in FIG. 3, the load of a driving amplifier becomes heavy and generation of heat from the amplifier becomes significant.